Numerical simulations of electrostimulation of frog gastrocnemius muscles have been carried out for pulse durations in the nanosecond regime. There are a number of potential advantages in using ultrashort pulses for electrical stimulation, and no previous electrostimulation work in the submicrosecond regime has been reported. A time-dependent, three-dimensional analysis model was developed and implemented for two cases: 1) direct stimulation via electrode contact and 2) indirect excitation through a saline-filled bath. The simulations yielded strength-duration (S-D) curves with pulse durations as short as 5 ns. Good agreement between the model predictions and experimental measurements was obtained. For example, with direct contact, a peak current of about 30 A was predicted for the shortest pulse; the measured value was 34 A. Calculations of the S-D curves for both direct and indirect stimulation yielded a good match with the available experimental data. A time constant of 160 μs was estimated; this value is indicative of a nerve-based response. The modeling also led to a demonstration of the nonthermal nature of electrostimulation with nanosecond pulses, even with an applied voltage of 5 kV. Finally, it was shown quantitatively that inhomogeneities in the nerve geometry and size can affect the S-D curve. For contact stimulation, the greatest potential for muscle twitching occurs at boundaries and within regions that have internal nonuniformity.